1. Field of the Invention
This invention generally relates to electrochemical cells and, more particularly, to a graphene-doped, carbohydrate-derived hard carbon (G-HC) composite and associated synthesis method, for use as an electrode in an alkali metal-ion battery.
2. Description of the Related Art
Modern rechargeable lithium battery technology has triggered the portable electronics revolution owing to high power density and long cycle life. The secondary lithium battery consists of a cathode (positive electrode) and anode (negative electrode), separated by a lithium ion (Li+)-permeable membrane. A solution or polymer rich in lithium ions is employed to permit the facile (readily occurring) migration of lithium ions between the positive and negative electrodes. Common positive materials include transition metal oxides such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4) and lithium iron phosphate (LiFePO4), among others. Li+ can travel within the interstitial space(s) of these materials both freely and reversibly. In general, metallic lithium, alloys, and carbonaceous materials can be utilized as the negative electrode. When the rechargeable lithium battery does not include a metallic electrode, it is referred to as lithium-ion battery (LIB).
During the discharge process in a lithium-ion battery, lithium ions are extracted from the negative electrode and subsequently inserted into the positive electrode. At the same time, electrons pass through an external circuit from the negative electrode to the positive electrode to generate electric power. During the charge process, ions and electrons move along the reverse directions and are ultimately restored to their original locations.
Despite the fact that LIBs have been employed successfully over a broad range of commercial applications, the issue of lithium supply, as it applies to both the strain on natural reserves and potential fluctuations in price, has motivated the development of a low-cost, rechargeable battery technology as an alternative to LIB. In light of this, sodium-ion batteries (NIBs) have increased in popularity due primarily to the fact that sodium has similar properties to lithium, but also boasts the benefits of both a potentially lower cost and virtually unlimited availability.
Although metallic sodium (Na) would make an ideal anode choice for sodium battery, its practical application is severely limited by safety concerns including reactivity, flammability, formation of dendritic structures during charge/discharge, and low melting point (<100° C.). In light of these potential hazards, non-sodium metal anodes appear to be more viable options for sodium battery applications. Currently, three main classes of materials have emerged as promising candidates for non-sodium metal anodes including: (1) metals (alloys with Na), (2) metal chalcogenides (metal oxides and sulfides), and (3) carbonaceous materials.
According to calculations, tin (Sn) and lead (Pb) can alloy 3.75 Na atoms, which correspond to capacities of 847 milliamp hours per gram (mAh/g) and 485 mAh/g for Sn and Pb, respectively [1]. Xiao et al. described a Sn/antimony (Sb) alloy demonstrating a reversible capacity of 544 mAh/g with 80% capacity retention after 50 cycles [2]. Overall, although it has been shown that Sn, Sb, and Pb provide high capacity anodes upon alloy formation with Na, large volume changes associated with the alloying process deteriorate the electrode integrity, and ultimately degrade battery performance.
Sun et al. reported a thin film antimony tetroxide (Sb2O4) anode for NIB that exhibited a large reversible capacity (896 mAh/g) originating from alloying/dealloying and oxidation/reduction processes of Sb [3]. Spinel materials such as Cobalt(II,III) oxide (Co3O4) and lithium titanate (Li4Ti5O12) showed similar behaviors [4]. These materials provided discharge and charge voltages of ˜0.5 V and ˜1.0 V, respectively, in half cell configurations with Na metal counter electrodes. Furthermore, the reversible capacities were ˜350 mAh/g for Co3O4 and ˜100 mAh/g for Li4Ti5O12. Nickel cobaltite (NiCo2O4) was shown to react with sodium ions (Na+) from 1.2 V to 0 V (vs Na/Na+), which were removed between 0.3 V and 1.5 V (vs Na/Na+) and for which a reversible capacity ˜200 mAh/g was demonstrated [5]. As an alternative to metal oxides, nickel sulfide (Ni3S2) was explored as a potential candidate for NIB anodes, for which discharge from ˜1.1 V to 0.3 V and charge from 1 V to 1.8 V (vs Na/Na+) with a reversible capacity ˜250 mAh/g was shown [6].
In general, carbonaceous materials consist of three allotropes including diamond, graphite, and buckminsterfullerene. Within the context of metal-ion batteries, only graphite and its disordered forms qualify as practical anode materials. Graphite features a typical layered structure into/from which lithium ions (Li+) can reversibly intercalate/de-intercalate. Unfortunately, larger Na+ (and potassium ions, K+) species do not readily insert into the layered structure as evident by low corresponding capacities. However, upon the action of specific treatments, carbonaceous materials can be rendered as amorphous. Based upon their respective crystallinities, amorphous carbons may be further classified as “soft” (graphitizable) and “hard” (non-graphitizable) carbons.
In general, Na+ intercalation into hard carbon (HC) is considered to be a two-step process consisting of: (1) Na+ insertion between parallel layers of graphene (high voltage region), and (2) intercalation of Na into pores of HC (low voltage region). Although the characteristic properties of HC are favorable for NIB applications, the corresponding capacities are often too low to be practical. Within the context of NIB applications, the formation of a solid electrolyte interface (SEI) on the surface of HC during the first cycle (Na+ ion insertion into HC) consumes a significant proportion of Na+, which results in a high irreversible capacity. A comprehensive treatment of the reactivity of carbon materials towards Li+ and Na+ under various experimental parameters has been previously provided [7-11].
Currently, the preparation of “state-of-the-art” HC materials requires thermal anneal at temperatures exceeding 1400° C. in order to decrease the surface area, the latter of which is known to be beneficial in terms of reducing irreversible capacity (SEI formation). Although it has been reported that the specific surface area (SSA) of HC decreases with an increase in pyrolysis temperatures, such extreme processing temperatures will not only increase production costs, but also reduce the specific capacity of HC.
It would be advantageous if alternative processes existed for synthesizing a graphene-doped, carbohydrate-derived hard carbon (G-HC) at a low pyrolysis temperature, with a low specific surface area (SSA).    (1) V. L. Chevrier and G. Ceder, “Challenges for Na-ion Negative Electrodes”, Journal of the Electrochemical Society 2011, 158, A1011-A1014.    (2) L. Xiao, Y. Cao, J. Xiao, W. Wang, L. Kovarik, Z. Nie, and J. Liu, “High Capacity, Reversible Alloying Reactions in SnSb/C Nanocomposites for Na-ion Battery Applications”, Chemical Communications 2012, 48, 3321-3323.    (3) Q. Sun, Q-Q. Ren, H. Li, and Z-W. Fu, “High Capacity Sb204 Thin Film Electrodes for Rechargeable Sodium Battery”, Electrochemistry Communications 2011, 13, 1462-1464.    (4) Y. Kuroda, E. Kobayashi, S. Okada, and J-I. Yamaki, “Electrochemical Properties of Spinel-type Oxide Anodes in Sodium-Ion Battery”, 218th ECS Meeting, Abstract#389.    (5) R. Alcantara, M. Jaraba, P. Lavela, and J. L. Tirado, “NiCo2O4 Spinel: First Report on a Transition Metal Oxide for the Negative Electrode of Sodium-Ion Batteries”, Chemistry of Materials 2002, 14, 2847-2848.    (6) J-S. Kim, G-B. Cho, K-W. Kim, J-H. Ahn, G. Wang, and H-J. Ahn, “The Addition of Iron to Ni3S2 Electrode for Sodium Secondary Battery”, Current Applied Physics 2011, 11, S215-S218.    (7) M. M. Doeff, Y. Ma, S. J. Visco, and L. C. De Jonghe, “Electrochemical Insertion of Sodium into Carbon”, Journal of the Electrochemical Society 1993, 140, L169-L170.    (8) X. Xia and J. R. Dahn, “Study of the Reactivity of Na/Hard Carbon with Different Solvents and Electrolytes”, Journal of the Electrochemical Society 2012, 159, A515-A519.    (9) E. Buiel and J. R. Dahn, “Reduction of the Irreversible Capacity in Hard-Carbon Anode Materials Prepared from Sucrose for Li-Ion Batteries”, Journal of the Electrochemical Society 1998, 145, 1977-1981.    (10) E. Buiel, A. E. George, and J. R. Dahn, “On the Reduction of Lithium Insertion Capacity in Hard-Carbon Anode Materials with Increasing Heat-Treatment Temperature”, Journal of the Electrochemical Society 1998, 145, 2252-2257.    (11) W. Xing and J. R. Dahn, “Study of Irreversible Capacities for Li Insertion in Hard and Graphitic Carbons”, Journal of the Electrochemical Society 1997, 144, 1195-1201.